Controversial plans to put armed guards on British passenger planes were in disarray last night after British Airways effectively refused to fly with them aboard because it would mean there was a 'significant threat' to passengers.

An internal BA memo obtained by The Observer makes clear that executives are deep-seatedly opposed to the scheme unveiled by the Government last week as a vital new step to protect aircraft against hijackers.

The memo - sent on Friday from Mike Street, BA's operations director - said the airline 'would not operate a single flight unless we were satisfied totally that it was safe to do so'. The sky marshals will be deployed only on flights where there has been a specific warning, prompting some pilots to voice concerns about security that a guard may be unable to prevent.

He added: 'If there is security information about a particular flight that gives us cause for concern, then we will not operate that flight. That remains our policy regardless of the Government's capability to deploy armed police officers.'

A spokesman for the British Air Line Pilots Association (Balpa), which also opposes the policy, said BA's statement was expected to be followed by other airlines and would rule out the use of sky marshals, as proposed.

'We now believe sky marshals will never fly,' said the spokesman. 'If you're told there's a perceived risk, you're not going to run it. No one in their right mind would say "Don't worry, we'll put sky marshals aboard it".'

Alistair Darling, the Transport Secretary, has refused to give details of the sky marshals, but Balpa said it was told armed guards would be used only when a specific threat had been made.

Because the marshals would not be used at random as a general deterrent - as in the US and Israel - it is also thought that only 24 are planned. They are likely to be former police officers and would use low-velocity bullets so as not to pierce the fuselage.

Critics condemned the plans as unworkable, with too few sky marshals. Simon Hughes, Liberal Democrat spokesman for London, said Ministers were 'trying to sound tough'. 'I think it was not token at the beginning, they were trying to cover all bases and thought this would do that,' he added. 'Then they have been pressured into doing something which is the minimum they could do to keep the States happy. To that extent, it's token.'

Balpa said passengers had been misled. In France, for example, every flight to the US now carries between two and six sky marshals, depending on how full they are.

'The [British] public seem to have the impression there will be one on every plane: it's nothing like that,' said the Balpa spokesman.

A Department for Transport official said he could not comment on when sky marshals would be used, how many would be recruited, how they would be trained, how they would operate, or any other details. 'We don't discuss that,' he said.

Balpa has already signed a deal over sky marshals with Virgin and could do a deal with BA this week. The union will meet government officials tomorrow in a bid to strike a national agreement instead. Under a deal with Virgin, the airline has promised its pilots will be introdcued to marshals used on their planes, and told where the guard is sitting. The pilots will be in control at all times, including emergencies.

Airlines or the Government will have to take responsibility for any accident on board and for pilots' insurance if they are refused personal cover, as some US pilots have been, said Balpa.

The union wants better land-based security instead. 'The Twin Towers [terrorist attacks in New York in 2001] happened not because there were no sky marshals but because there was lax security on the ground,' added the spokesman.